1. Field of the Invention
The present invention relates to a motor for rotating a rotor by switching the state of energization of coils, and more particularly to a motor provided with a position detecting unit for detecting a rotational position of a rotor.
2. Description of the Related Art
A stepper motor has a feature that it switches the state of energization of coils, whereby it is capable of rotating a rotor through an angle of rotation set for each step, and hence it is capable of easily performing position control even without a unit for detecting a rotational position of the rotor. For this reason, in driving the stepper motor, a drive mode is generally employed in which the state of energization of the coils is switched at predetermined time intervals by open-loop control.
However, in a case where the stepper motor is driven at a high speed or under a high-load condition, the rotor cannot respond to switching of the state of energization of the coils performed at the predetermined time intervals, which might cause a step-out of the motor.
To eliminate this fear, there has been proposed a stepper motor which is provided with a position detecting unit for detecting the rotational position of a rotor and has a drive mode in which high-speed drive can be achieved by execution of feedback control for switching the state of energization of coils according to an output from the position detecting unit. This type of stepper motor performs the feedback control based on rotor position information output from the position detecting unit, and therefore necessitates accurate rotor position information.
The above-mentioned type of stepper motor has been disclosed e.g. in Japanese Patent No. 03517548. In the stepper motor disclosed in Japanese Patent No. 03517548, an electric element is disposed between teeth of an outer yoke in opposed relation to the outer peripheral surface of a hollow cylindrical magnet which is magnetized in a circumferentially divided fashion, and the rotational position of the magnet (rotor) is detected by the electric element.
For detection of the rotational position of a magnet (rotor) magnetized in a circumferentially divided fashion as in the stepper motor disclosed in Japanese Patent No. 03517548, it is a common practice that a position detecting unit is circumferentially disposed in opposed relation to the magnet.
Further, a 2 phase driver stepper motor is generally provided with two position detecting units so as to facilitate rotational direction control. The two position detecting units are arranged side by side in the rotational direction of a magnet (rotor) such that they are opposed to the magnet surface in a manner shifted by ½ of a rotational angle corresponding to one pole (90 degrees in terms of electrical angle).
Based on rotor position information which is output from the two position detecting units to a motor controller, the motor controller switches the state of energization of coils according to the two phases of the stepper motor, whereby it is possible to perform feedback control of the 2 phase driver stepper motor.
If the two position detecting units are displaced in relative position from each other, it is difficult to accurately feed-back the rotor position information to the motor controller. As a solution to this problem, the two position detecting units (position detecting sensors) are provided in a single chip, and such single-chip position detecting sensor products are generally distributed on the market.
FIG. 12 is a view of a magnet and a position detecting sensor used in a stepper motor according to the related art.
FIG. 12 schematically shows only the magnet, denoted by reference numeral 101, and the position detecting sensor, denoted by reference numeral 102, as viewed axially from the stepper motor. The magnet 101 is formed into a hollow cylindrical shape, and is magnetized to have alternately different poles (a total of eight poles, i.e. four N poles and four S poles) in a circumferential direction thereof. The position detecting sensor 102 has a first sensor part 102a and a second sensor part 102b integrally formed in a single chip (package), and is disposed in opposed relation to the outer peripheral surface of the magnet 101.
When the first sensor part 102a of the position detecting sensor 102 is opposed to the boundary between an S pole and an N pole of the magnet 101, the second sensor part 102b is opposed to the center of the N pole of the magnet 101. The position detecting sensor 102 is thus disposed in a manner opposed to the magnet 101 such that an angle formed by the first sensor part 102a and the second sensor part 102b about the axis of a rotor rotating shaft becomes equal to ½ of a rotational angle corresponding to one pole (90 degrees in terms of electrical angle).
In short, signals output from the respective two position detecting units (the first and second sensor parts 102a and 102b of the position detecting sensor 102) are formed to have a phase difference of 90 degrees in terms of electrical angle. Therefore, by switching coil energization timing according to the positive or negative state of each of the signals, it is possible to obtain a high-efficiency stepper motor.
However, the position detecting sensor having two sensor parts built in a single chip as shown in FIG. 12 is on the market only in few types in respect of spacing between the two sensor parts. For this reason, when it is desired to obtain an appropriate spacing between the two sensor parts of the position detecting sensor and the magnetic poles of the stepper motor, it is only possible to select from the following two methods: a method of mechanically adjusting the distance between the position detecting sensor and the magnet which are opposed to each other, and a method of producing a special position detecting sensor in which spacing between the two sensor parts is customized.
However, in the case of the method of mechanically adjusting the distance between the position detecting sensor and the magnet which are opposed to each other, if the distance is adjusted by causing the position detecting sensor to get closer to the outer peripheral surface of the magnet, there is a fear of the position detecting sensor being brought into contact with the outer peripheral surface of the magnet. On the other hand, if the distance is adjusted by causing the position detecting sensor to get further from the outer peripheral surface of the magnet, the output from the position detecting sensor decreases, which results in an increased detection error due to lowered sensitivity of the position detecting sensor.
Further, the method of producing a special position detecting sensor in which spacing between the two sensor parts is customized is very costly. Especially, it is not practical under the constraints of costs to produce a position detection sensor customized in spacing between the two sensor portions each time a stepper motor is developed which is different in the number of poles and outer dimension of magnets.
A further alternative to the above, there is a method in which spacing between the two sensor parts is electrically adjusted as phase adjustment (electrical adjustment). However, when a mutual position error between the two sensor parts, an error in mounting of the package, and so forth are considered, it is necessary to perform the electrical adjustment on a stepper motor-by-stepper motor basis. The drive circuit is usually provided separately from the stepper motor. Therefore, it is required to perform electrical adjustment on a stepper motor-by-stepper motor basis to thereby determine an electrical adjustment value for each stepper motor, and then write the adjustment value in a drive circuit when the stepper motor is integrated into an apparatus.